Yes, you may change cards from the command line and you may override the default (which is slot 9).This comment
from earlier covers basic command line usage of the slots. There are more cards now (I think 'mess -listslots maciix' will show the possible cards).
All of the working cards are video cards; there is work in progress on an Ethernet card but it's not working yet. You may have up to 4 video cards active at a time right now due to core limitations, or on the IIci you can have built-in video plus 3 video cards. On Windows you can map the multiple video cards to multiple physical monitors; on SDL you can have them tiled 4-across or 2x2 by pressing Tab and going into Video Options.